JP4640909B2 - Production of high-strength steel articles by controlling inclusions during melting - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to the manufacture of high strength steel articles, and more particularly to the control of aluminum-oxygen based content in the final article during and after melting.
[0002]
[Prior art]
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, pressurized by an axial compressor, and mixed with fuel. The mixture is combusted and the resulting hot combustion gas passes through the axial turbine. The gas stream rotates the turbine by contacting the airfoil portion of the turbine blade, which in turn provides output to the compressor. Hot exhaust gas is ejected from the rear of the engine, driving the engine and aircraft forward.
[0003]
Various stages of the compressor and turbine, and if provided, are mounted on shafts and shaft segments that extend along the centerline of the gas turbine engine and are connected together. Some of the shafts are made of high strength steel. These shafts need to have excellent strength, but at the same time, importantly, because of the various forms of loading on the shafts, they must have excellent low cycle fatigue life under torsion. Don't be.
[0004]
Traditionally, shafts have been manufactured from maraging steel containing titanium nitride precipitates. After studies have shown that these precipitates limit the low cycle torsional fatigue life, a series of high strength, low titanium maraging steels has been developed. These steels are strengthened by adding about 0.5 weight percent to about 1.3 weight percent of aluminum in place of the titanium addition in the previous generation steel. These high aluminum steels are described in Patent Document 1, and the disclosure of this patent is incorporated herein by reference. The steel described in this document provides a significantly improved shaft fatigue life.
[0005]
[Patent Document 1]
US Pat. No. 5,393,488
[Problems to be solved by the invention]
However, there is still room for improvement. There is a continuing need to further increase the fatigue life of the steel described in Patent Document 1 without adversely affecting strength, durability and other steel properties, and without requiring significant changes in processing parameters. It is.
The present invention fulfills this need and provides further related advantages.
[0007]
The present invention provides an improved melting and casting technique for high aluminum steels such as those described in US Pat. The new approach reduces the presence of inclusion clusters based on the aluminum-oxygen composition. Such a cluster, when it exists, will serve as a fatigue failure occurrence site. Other desired mechanical properties of the steel are not adversely affected by the technique of the present invention. Steel produced by the techniques of the present invention has application as a shaft for gas turbine engines, and other applications are found as well.
[0008]
[Means for Solving the Problems]
The method for producing steel articles is 10% to 18% nickel, 8% to 16% cobalt, 1% to 5% molybdenum, less than 0.5% aluminum. And an iron-base alloy having from 1% to 3% by weight of chromium , after which the alloy is melted into a melt in a vacuum furnace , after which the melt is added to a melt in an amount greater than 200 ppm by weight. addition of calcium in 1 amount, then aluminum was added to the melt, the aluminum content of the melt 0. Increasing to an aluminum content greater than 5 weight percent, and then casting the melt into a casting.
[0009]
The initially prepared iron-based alloy has an aluminum content that is desirably less than about 0.5 weight percent, and preferably less than about 0.1 weight percent, for use in the preferred melting procedure. In one embodiment, the prepared iron-base alloy comprises from about 10 weight percent to about 18 weight percent nickel, from about 8 weight percent to about 16 weight percent cobalt, from about 1 weight percent to about 5 weight percent. Molybdenum, less than about 0.5 weight percent (preferably less than about 0.1 weight percent) aluminum, and about 1 weight percent to about 3 weight percent chromium with the balance being iron and other minor components It is what has. The addition of aluminum desirably increases the aluminum content of the melt from about 0.5 weight percent to about 1.3 weight percent. In one embodiment, the final casting desirably includes from about 10 weight percent to about 18 weight percent nickel, from about 8 weight percent to about 16 weight percent cobalt, from about 1 weight percent to about 5 weight percent. About 0.5 weight percent to about 1.3 weight percent aluminum, about 1 weight percent to about 3 weight percent chromium, about 0.3 weight percent carbon, about 0.1 weight percent Contains less titanium with the balance having iron and other minor components.
[0010]
In the normal case, the initially prepared iron-base alloy has a relatively high carbon content, usually greater than about 0.3 weight percent. The initially prepared iron-based alloy is melted in a vacuum furnace and the pressure is gradually reduced, during which a carbon-oxygen chemical reaction (called “carbon boiling”) takes place, resulting in a weight ratio of less than 10 ppm. It is preferable that the oxygen content of the melt is reduced. Next, preferably, the first added calcium in an amount greater than about 200 ppm by weight is added. Optionally but preferably, at the same time as adding the aluminum, an additional step is performed in which the second added calcium is added to the melt, desirably in an amount of about 100 to about 200 ppm by weight. Optionally but preferably, an additional step is performed after the step of adding aluminum and prior to the casting step, preferably adding a third added calcium in an amount of about 50 to about 150 ppm by weight to the melt. The The calcium addition is preferably performed in the form of an alloy such as NiCa. Aluminum-oxygen based clustered inclusions, when they are present, will impair the low cycle fatigue performance of steel articles, but calcium addition was clustered in the absence of calcium addition. During the period in which inclusions are to be formed, the melt is reduced, reducing the chance of formation of this clustered inclusion.
[0011]
The steel of the present invention is usually not used in the as-cast state and is generally machined (including machining and / or heat machining). As the most interesting application, castings are machined into gas turbine engine shafts.
[0012]
In a preferred embodiment, a method for manufacturing a steel article comprises providing an iron-based alloy having a carbon content greater than about 0.3 weight percent and an aluminum content less than about 0.1 weight percent, after which Melting the alloy into a melt in a vacuum furnace. Melting the alloy includes gradually reducing the pressure in the vacuum furnace to cause carbon boiling in the melt, thereby reducing the oxygen content of the melt to a value less than about 10 ppm by weight. . Thereafter, a first added calcium in an amount greater than about 200 ppm by weight is added to the melt. The method further includes simultaneously adding aluminum to the melt to increase the aluminum content of the melt to a value greater than about 0.5 weight percent, in an amount of about 100 to about 200 ppm by weight. Adding the second added calcium to the melt. Thereafter, it is preferred to add the third added calcium to the melt, preferably in an amount of about 50 to about 150 ppm by weight. The indicated amount of calcium addition is that of a typical case. The amount added can be varied as needed based on the amount of oxygen actually present in the melt, and this amount of oxygen can be readily measured by conventional real-time techniques. Other possible chemical oxidants can be used in place of calcium. The melt is then cast and the casting is machined. Compatible features described anywhere in this specification are applicable to this embodiment.
[0013]
In another embodiment, a method for manufacturing a steel article melts an iron-based alloy having an aluminum content of less than about 0.5 weight percent, while the oxygen content of the melt is about Including reducing to a value less than 10 ppm. Reducing the oxygen content includes adding a reducing agent such as calcium to the melt. Aluminum is added to the melt and is increased until the aluminum content of the melt is greater than about 0.5 weight percent, after which the melt is cast and cast. Preferably, the melt initially has an aluminum content less than about 0.1 weight percent and a carbon content greater than about 0.3 weight percent. Compatible features described anywhere in this specification are applicable to this embodiment.
[0014]
The composition described in U.S. Patent No. 6,099,056 achieved a significant improvement on the low cycle fatigue life of the steel by reducing the titanium content of the steel, thereby reducing the presence of titanium nitride inclusions. It was observed that these inclusions cause fatigue failure. The steel described in U.S. Pat. No. 6,056,048 is strengthened by the addition of aluminum, on the order of about 0.5 weight percent to about 1.3 weight percent, which is a relatively large amount for the steel. We have observed that fatigue failure in final articles cast and processed with this high aluminum steel and similar steels begins with aluminum-oxygen based clustered inclusions (sometimes referred to as “rafts”). Found. These aluminum-oxygen based clusters were traced back to the melting procedure. When the high aluminum steel alloy is melted prior to casting, the aluminum forms an aluminum-oxygen based inclusion cluster in the molten steel. These inclusion clusters remain in the casting and then in the final machined product leading to premature fatigue failure.
[0015]
One approach that may alleviate this problem is to add calcium to the high aluminum steel melt just before casting. However, this study showed that calcium addition to the high aluminum melt just prior to casting was not sufficient to avoid the presence of aluminum-oxygen based content in the final product.
[0016]
Rather, the problem of inclusion is that a melt with a relatively low aluminum content is first prepared, then calcium is added prior to the rest of the aluminum addition, and the aluminum content is reduced to the desired amount, typically in the final product. Has been found to be significantly reduced by going from about 0.5 weight percent to about 1.3 weight percent. Optionally but preferably, calcium is added in the same manner as the aluminum addition. The increased calcium content in the melt reduces the free oxygen used to become aluminum-oxygen based clusters. Calcium reacts with free oxygen in the melt to form a product, such as calcium oxide and / or calcium aluminate, which is no longer free. Following the addition of aluminum, further optional calcium can be added to react with oxygen that may be introduced into the melt during processing of the melt prior to casting. In the final product, the concentration of aluminum-oxygen based clusters is reduced. A reducing agent functionally equivalent to calcium may be used as well.
[0017]
The improved low cycle fatigue life of the final article produced from steel is a result of this change in approach. Other features and advantages of the present invention will become apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. However, the technical scope of the present invention is not limited to this preferred embodiment.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an example of a steel article 20 that can be produced by the technique of the present invention. Article 20 is preferably a shaft used in a gas turbine engine. However, the use of the present invention is not limited to this article.
[0019]
FIG. 2 shows a block diagram that constitutes a preferred approach for practicing the present invention.
As indicated by reference numeral 30, an iron-based alloy is prepared. Iron-based alloys have more iron than any other component. The iron-based alloy contains aluminum present in relatively small amounts of less than about 0.5 weight percent, preferably less than about 0.1 weight percent. Other ingredients are usually present. In a preferred form, the iron-base alloy comprises about 10 weight percent to about 18 weight percent nickel, about 8 weight percent to about 16 weight percent cobalt, about 1 weight percent to about 5 weight percent molybdenum, about 0 weight percent. Less than 5 weight percent aluminum and from about 1 weight percent to about 3 weight percent chromium. Carbon is typically present in the initially prepared iron-based alloy in an amount greater than about 0.3 weight percent, and the carbon content is reduced during the melting process as described below. Titanium is present in an amount less than about 0.1 weight percent, if present. The remainder of the composition is iron and possibly other components that are intentionally present and impurities. (Here all compositions are weight percentages unless otherwise stated).
[0020]
As indicated by reference numeral 32, the alloy is then melted. Melting is preferably accomplished in a vacuum furnace at a pressure that eventually reaches a value below about 50 micrometers pressure. The most preferred vacuum furnace is a vacuum induction melting furnace using a crucible made of aluminum oxide or magnesium oxide.
[0021]
As shown at 34 in FIG. 2, the melting process reduces the free oxygen content of the melt to a low level, resulting in a reaction with aluminum forming an aluminum-oxygen based clustered harmful content. There is almost no free oxygen available. In the preferred inventive approach, the free oxygen content is reduced by two main mechanisms. First, when the pressure in the vacuum chamber is reduced, the carbon and free oxygen react together to form gaseous carbon dioxide and carbon monoxide, which will bubble from the melt. This reaction and bubbling are in a stirred state and the expression “carbon boiling” is used. Carbon boiling does not occur appreciably if the aluminum content is too high, and for this reason, the aluminum content of the initially prepared melt is preferably less than about 0.1 weight percent . However, higher aluminum content may be used if other oxygen reduction techniques are used in the initial stages of melting. The free oxygen content of the melt is preferably less than about 10 ppm by weight at the end of stage 34.
[0022]
As shown at 36 in FIG. 2, a first additive chemical reducing agent, preferably calcium, is added to further reduce the oxygen content of the melt. The calcium addition is preferably in an amount greater than about 200 ppm by weight, and excess calcium is selected to react and bind with substantially all of the free oxygen in the melt. Calcium can be added in any practicable form that results in elemental calcium present in the melt. In developing the technique of the present invention, NiCa was used as a calcium source.
[0023]
As indicated at 38, aluminum is then added to the melt until the final desired aluminum content of the alloy is reached. The preferred aluminum content of the cast alloy is from about 0.5 weight percent to about 1.3 weight percent. The chemical composition of the other components of the melt can be adjusted to their desired final values at this point as well, based on the chemical analysis performed during the melting process.
[0024]
Preferably, the most preferred calcium as the second addition chemical oxidant is added to the melt simultaneously with the aluminum addition in step 38. The second added calcium is preferably from about 100 to about 200 ppm by weight.
[0025]
The first added calcium of stage 36 prior to the aluminum addition of stage 38 and the second added calcium simultaneously performed in stage 38 is to provide a chemical reducing agent in the melt, which is free from chemicals and free oxygen present in the melt. It reacts to become a compound such as calcium oxide or calcium aluminate. These compounds are difficult to cluster. Free oxygen that reacts with the excess aluminum added in step 38 to form an aluminum-oxygen-based species that is susceptible to clustering and ultimately produces undesirable clustered inclusions in the final casting product, No longer exists. In the absence of the processing approach of the present invention, such aluminum-oxygen based clusters are formed and become inclusions in the final product. This inclusion serves as an early fatigue fracture site.
[0026]
Free oxygen tends to diffuse into the melt under vacuum conditions in the vacuum melting furnace and even during the subsequent casting process, which can lead to the formation of aluminum-oxygen clusters. Thus, as shown in step 40 of FIG. 2, after the aluminum is adjusted to its final value in step 38, and before or during the casting process of step 42, the third addition calcium is added to the melt and whatever is present. It is preferable to chemically react with free oxygen. The third added calcium is preferably in an amount from about 50 to about 150 ppm by weight, and most preferably in an amount of about 100 ppm by weight.
[0027]
As indicated at 42, the melt is then cast and solidified. Any practicable fixed mold or continuous casting process can be used.
[0028]
Preferred alloys are forged alloys that are not used in the as-cast form.
Rather, as shown at 44, the casting is machined to the final desired shape, such as the shaft 20 of FIG. Machining 44 can include room temperature processing, higher temperature processing, or heat machining processing. The heat treatment can be used as necessary. Further details of a more preferred casting alloy processing technique can be found in US Pat. No. 5,393,488. The advantage of the technique of the present invention is that the same machining process can be used in this technique as in the conventional technique for generating articles.
[0029]
3 and 4 are ideal microstructures of the article 20. The microstructure of FIG. 3 is for a material produced according to the present invention to which first added calcium 36 has been added. The microstructure of FIG. 4 is for a material produced without the first added calcium 38 prior to the aluminum addition 38 and shows a product not included within the scope of the present invention. In this material of FIG. 4, there are aluminum-oxygen-based clusters 24 that act as inclusions that diffuse throughout the microstructure. These clusters 24 are very large and each typically has a planar area of several hundred square microns in the microstructure. The large clusters 24 may act as a cause of fatigue cracking, particularly as a cause of low cycle fatigue cracking in the final product. In contrast, in the fine structure of FIG. 3, fine particles 26 are present and distributed throughout the fine structure. The particulates 26 are not clustered to a sufficient size to act as a cracking site and to have a strong adverse effect on the low cycle fatigue properties of the final product.
[0030]
The method of the present invention was actually implemented. The results of a comparative test for this article with and without calcium addition are shown in FIG. The technique of the present invention using calcium addition is generally superior and achieves better fatigue results, particularly in the critical low cycle fatigue region towards the left hand of the graph.
[0031]
While specific embodiments of the present invention have been described in detail for purposes of illustration, various modifications and improvements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
[0032]
Reference numerals appearing in the claims are not intended to limit the scope of the invention but to facilitate understanding thereof.
[Brief description of the drawings]
FIG. 1 is a perspective view of a steel shaft according to the present invention.
FIG. 2 is a block flow diagram of a technique for carrying out the present invention.
3 is an ideal microstructure of steel prepared by the technique of FIG.
FIG. 4 is an ideal microstructure of a steel having a composition similar to that shown in FIG. 3 with no calcium added prior to aluminum addition.
FIG. 5 shows alternating pseudo-stress graphs as a cycle function until failure in low cycle fatigue with and without calcium addition.
[Explanation of symbols]
20 Steel articles (shaft)
24 clusters 26 fine particles

Claims (8)

10% to 18% nickel, 8% to 16% cobalt, 1% to 5% molybdenum, less than 0.5% aluminum, and 1% to 3% by weight Prepare an iron-based alloy with up to chromium, then
In a vacuum furnace, the alloy is melted into a melt, then
Adding a first added amount of calcium in an amount greater than 200 ppm by weight to the melt;
Adding aluminum to the melt, increasing the aluminum content of the melt until an aluminum content of greater than 0.5 wt.
Casting the melt into a casting;
A method for producing a steel article (20), characterized in that it comprises a step.   The method of claim 1, wherein the step of preparing the iron-based alloy comprises the step of preparing an iron-based alloy having an aluminum content of less than 0.1 wt%.   An addition that reduces the oxygen content of the melt to a value less than 10 ppm by weight at the same time as the step of melting the alloy and prior to the step of adding a first addition amount of calcium to the melt. The method according to claim 1, characterized in that a central step is performed.   The method of claim 1, wherein an additional step of adding a second addition amount of calcium to the melt is performed simultaneously with the step of adding aluminum. The method of claim 4 , wherein the second addition amount of calcium is in an amount of 100 to 200 ppm by weight. Preparing the iron-based alloy;
Melting the alloy into a melt in a vacuum furnace,
The step of melting the alloy comprising the steps of:
Gradually reducing the pressure in the vacuum furnace to cause carbon boiling in the melt, reducing the oxygen content of the melt to a value less than 10 ppm by weight,
To the melt, a first addition amount of calcium in an amount greater than 200 ppm by weight is added, and at the same time,
Aluminum is added to the melt, the aluminum content of the melt is increased to an amount greater than 0.5% by weight, and a second addition amount of calcium in an amount of 100 to 200 ppm by weight is added to the melt. And then
A third additive amount of calcium in an amount of 50 to 150 ppm by weight is added to the melt;
Casting the melt into a casting, then
Machining the casting,
The method of claim 1, comprising steps. The step of adding aluminum comprises adding sufficient aluminum to increase the aluminum content of the melt to an aluminum content of 0.5 wt% to 1.3 wt%. 7. A method according to claim 1 or claim 6 . The step of casting the melt into a casting;
The melt is cast to 10% to 18% nickel, 8% to 16% cobalt, 1% to 5% molybdenum, 0.5% to 1.3%. A casting having a composition comprising up to aluminum by weight, 1 to 3% by weight of chromium, up to 0.3% by weight of carbon, less than 0.1% by weight of titanium, the balance being iron and inevitable impurities. To
7. A method according to claim 1 or claim 6 , comprising steps.

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